Binders or binder systems for foundry cores and molds are well known. In the foundry art, cores or molds for making metal castings are normally prepared from a mixture of an aggregate material, such as sand, and a binding amount of a binder or binder system. Typically, after the aggregate material and binder have been mixed, the resultant mixture is rammed, blown, or otherwise formed to the desired shape or patterns, and then cured with the use of catalyst and/or heat to a solid, cured state.
Resin binders used in the production of foundry molds and cores are often cured at high temperatures to achieve the fast-curing cycles required in foundries. However, in recent years, resin binders have been developed which cure at a low temperature, to avoid the need for high-temperature curing operations which have higher energy requirements and which often result in the production of undesirable fumes.
One group of processes which do not require heating in order to achieve curing of the resin binder are referred to as "cold-box" processes. In such processes, the binder components are coated on the aggregate material, such as sand, and the material is blown into a box of the desired shape. Curing of the binder is carried out by passing a gaseous catalyst at ambient temperatures through the molded resin-coated material. In such processes, the binder components normally comprise a polyhydroxy component and a polyisocyanate component. These cure to form a polyurethane in the presence of a gaseous amine catalyst.
Another group of binder systems which do not require gassing or heating in order to bring about curing are known as "no-bake" systems. These "no-bake" systems also frequently employ an aggregate material, such as sand coated with a polyhydroxy component and a polyisocyanate component. In this case, the coated sand is usually mixed with a liquid tertiary amine catalyst just before the sand is placed into a holding pattern or core box, and the material is allowed to cure at ambient temperatures or slightly higher.
Although developments in resinous binder systems which can be processed according to the "cold-box" or "no-bake" processes have resulted in the provision of useful systems, such systems with urethane binders still exhibit certain disadvantages. For example, cores and molds made with these binders have relatively low hot strength. Low hot strength results in foundry cores and molds that are prone to casting defects such as scabs, erosion, and burn-in. These defects have limited the use of systems employing urethane binders in certain iron and steel casting applications. A reduction in these casting defects would be of great value to foundries.
Now it has been found, in accordance with this invention, that the use of certain modified polyhydroxy components in the "no-bake" and "cold-box" processes overcomes this deficiency and provides cores and molds with greater hot strength.